An engine for an airplane which mounts a propeller on one end of a crankshaft is known by JP-A-2002-213302, and a vibration prevention structure which suppresses the vibrations of a crankshaft is disclosed in JP-A-2003-293701.
Here, as disclosed in the above-mentioned JP-A-2002-213302, the crankshaft which mounts the propeller on one end thereof has a large possibility of generating torsional vibrations. Particularly, the vibrations are increased at a portion remote from the propeller and hence, it is necessary to suppress the torsional vibrations. On the other hand, in the engine which is disclosed in the above-mentioned JP-A-2003-293701, a damper weight is mounted on a crank web of the crankshaft to obtain a vibration prevention effect. In applying such vibration prevention structure to the crankshaft which mounts the propeller on one end thereof, the vibration reduction effect on an end portion of the crankshaft opposite to the propeller is small and, at the same time, with respect to an operation which is performed at the time of changing the vibration suppression structure in conformity with properties of the propeller, a large-scale operation such as dissembling of the crank case or the like becomes necessary and such an operation can not be performed easily.
The present invention has been made under such circumstances and it is an object of the present invention to provide the vibration prevention structure in an engine which can effectively suppress the vibrations of a crankshaft at an end portion thereof opposite to a propeller and, at the same time, can easily perform a change of the vibration suppression structure in conformity with properties of the propeller.